RFID has become established in a wide range of applications for the detection and identification of items, allowing substantial amounts of data to be read at greater range than other technologies. Of particular interest is the high frequency (UHF) passive RFID system which promises to offer read ranges of the order of ten metres using tags which do not require their own power source. Improved techniques for longer range reading of a RFID tag in turn stimulates a desire for improved tag location techniques. However RFID tags are typically difficult to locate accurately because of multipath fading, and this can significantly restrict their use in applications where accurate location sensing is important.
In order for a passive UHF RFID tag to be successfully read, it should receive sufficient radio frequency (RF) power for its internal logic to be activated and transmit back to the reader with sufficient signal-to-noise ratio (SNR). This requirement sets limits on the maximum tag range. However, due to the narrowband nature of the signals, fading effects in real environments generate large variations in the free space loss of both up- and downlink directions and can prevent successful reading of the tag, even well within the maximum read range. Therefore in order to fully deploy these passive UHF RFID tags in real applications, robust reading techniques are required for long range conditions.
By expanding the range of view of a single RFID reader, as well as improving the likelihood of successful tag detection, one can envisage RFID systems with wide coverage areas as opposed to the portal systems currently in use today, where sensitivity constraints require the objects to pass close to the reader antennas for detection. In a portal system however, the location of a tagged object can be inferred from the fact that it has passed close enough to the reader to be read. In a wide area RFID system, the simple reading of a tag will not provide sufficient location resolution for many applications. As a result interest has also arisen in being able to estimate the location of the tag in such systems. Due to the complex multipath environment commonly encountered in RFID implementations, fading and nulls result in the RSSI being only a weak function of range and hence providing location in passive RFID system is a major challenge.
Several studies have been undertaken to enhance passive UHF RFID system performance. However, standard RFID systems currently cannot prevent errors (i.e. 100% probability of a successful read). By way of example, “The RF in RFID—passive UHF RFID in practice” by Daniel M. Doubkin proposes a number of ways of improving SNR: The author suggests that inclusion of a 90° phase shift either the in in-phase (I) or quadrature (Q) channel in the conventional direct-conversion I/O demodulator improves the SNR of the tag backscattered signal since the phase of the backscattered signal is unpredictable due to its dependent on the distance from the tag.
By way of further example, Mojix (http://www.mojix.com/) has a passive UHF RFID system with phased array of antennas (i.e. the antennas are in the near field region of one another). This allows phased array techniques to be employed, for example digital beam forming steering to maximise the link budget. This enables improved receiver sensitivity and transmitters which provide radio frequency (RF) signals in the industrial, scientific and medical (ISM) band (902 MHz and 928 MHz) for activating the tags. Using this scheme a 99.9% tag detection is claimed. Details can be found, for example in: WO2007/094868, WO2008/118875 and WO2008/027650. Further background can be found in: EP2146304 and in US 2008/0024273.
The EPC global UHF Class 1 Generation 2 RFID protocol standard allows frequency hopping spread spectrum (FHSS) technique in the US and listen-before-talk technique in the UK to overcome interference in multiple- and dense-interrogator environment [EPCglobal Specification for RFID Air Interface, online available: http://www.epcglobalinc.org/standards/uhfc1g2/uhfc1g2 1 2 0-standard-20080511.pdf;] [EPCglobal Class Gen 2 RFID Specification, Alien, online available: http://www.rfidproductnews.com/whitepapers/files/AT wp EPCGlobal WEB.pdf].
To date, a number of location schemes for passive RFID have been proposed. The most common techniques are based on received signal strength indicator (RSSI) location algorithms:
Hatami and K. Pahlavan, “A Comparative Performance Evaluation of RSSI-Based Positioning Algorithms Used in WLAN Networks,” in Proc IEEE Wireless Communications and Networking Conference, pp. 2331-2337, 2005]; [A. Hatami and K. Pahlavan, “Comparative Statistical Analysis of Indoor Positioning Using Empirical Data and Indoor Radio Channel Models,” in Proc IEEE CCNC 2006, pp. 1018-1022, 2006]; [B. Xu and W. Gang, “Random Sampling Algorithm in RFID Indoor Location System,” in Proc Third IEEE International Workshop on Electronic Design, Test and Applications, pp. 168-176, 2006]; [J. Zhao, Y. Zhang and M. Ye, “Research on the Received Signal Strength Indications Algorithm for RFID System,” in Proc ISCIT 2006, pp. 881-885, 2006]; [F. Guo, C. Zhang, M. Wang and X. Xu, “Research of Indoor Location Method Based on the RFID Technology,” in Proc 11th Joint Conference on Information Sciences 2008, 2008]; [A. Chattopadhyay and A. Harish, “Analysis of UHF passive RFID tag behaviour and study of their applications in Low Range Indoor Location Tracking,” IEEE Antennas and Propagation Society International Symposium, pp. 1217-1220, 2007.
However due to the complex multipath environment commonly encountered in RFID implementations, fading and nulls result in the RSSI being only a weak function of range.
In active RFID, radar and other wireless systems, a number of powerful location techniques such as time difference of arrival (TDOA) and phase difference of arrival (PDOA) are used. Due to narrow bandwidth available for passive RFID, the TDOA technique cannot be applied to locating passive tags. This is because the narrow bandwidth gives insufficient time resolution for typical RFID ranges.
The PDOA technique can be applied to passive RFID. However, this only works well for line-of-sight communication (i.e. in free-space). In real environments, the PDOA technique suffers from multi-path fading as the fading introduces ambiguities in phase measurements (the phase shift of a direct path returned signal cannot accurately be determined from the sum of multi-path signals. This challenge is also addressed by Pavel V. Nikitin et al, in “Phase Based Spatial Identification of UHF RFID Tags”, IEEE RFID 2010.
However, a number of researchers claim to estimate range using this technique. For example, Ville Viikari et al, in “Ranging of UHF RFID Tag Using Stepped Frequency Read-Out”, IEEE RFID 2010 and Xin Li et al, in “Multifrequency-Based Range Estimation of RFID Tags”, IEEE RFID 2009. By way of further example, a number patents also claim to estimate location based on PDOA. For example, Alien technology and Symbol technologies outlines location technique based on PDOA technique as described in WO 2006/099148 A1 and AU 2010200808 A1 respectively. However, to our knowledge this technique only works reliably for up to a short range (i.e. up to 3 or 4 m) due to multi-path fading.
A technique for transmitting signals at a plurality of antenna polarisations for improved reading of an RFID tag is described in US 2010/0052857. Mojix also outlines a location approach using PDOA technique over a phased array antenna system as described in WO2009151778 (A2).
However there is a need for improved techniques for reading in particular UHF passive RFID tags, and for locating such tags.